Method for producing transition metal oxide fine particles

ABSTRACT

Disclosed is a method for producing transition metal oxide fine particles having a size smaller than several micrometers (μm), and more preferably, having a size of several hundred nanometers (nm). To this end, the method for producing transition metal oxide fine particles of the present invention comprises dissolving a transition metal oxide in a strongly basic aqueous solution, and titrating same with a strongly acidic aqueous solution, thereby precipitating transition metal oxide fine particles.

BACKGROUND Technical Field

The present disclosure relates to a method for producing transition metal oxide fine particles having a size smaller than several micrometers (μm), preferably, a size of hundreds of nanometers (nm).

Background Art

When transition metal oxide coating is applied to a base material, the base material may have antibacterial and catalytic properties. Transition metal oxide quickly changes the surface of the base material to acid when meeting moisture in the air, thereby having antibacterial properties that inhibit the growth of bacterial and destroy bacterial. In addition, the transition metal oxide may have the catalytic property of absorbing and oxidizing some odorous substances to change them into odorless compounds.

To maintain the material properties such as polymers as the base material, even if the antibacterial and catalytic properties are given to the base material by using the transition metal oxide, the transition metal oxide should be particles having a relatively large surface area. For example, porous fine particles having a size of several micrometers to several hundreds of nanometers may be used.

A physical grinding (e.g., milling) method may be considered as a method of manufacturing transition metal oxide fine particles. However, when using such a physical grinding method, it is very difficult to control the particle size of the transition metal oxide a size of several micrometers to several hundreds of nanometers.

Korean Patent No. 10-2004-0082246 (published on Sep. 24, 2004) cited herewith discloses a method for manufacture of transition metal oxide fine particles using coprecipitation and impregnation, particularly, an antibacterial and deodorizing multi-component metal oxide fine particles. However, the cited document uses a ball mill method to manufacture the fine particles and it has the limitations of the physical grinding method, and also discloses no specific method of controlling the size of the fine particles.

DESCRIPTION OF DISCLOSURE Technical Problems

Accordingly, an object of the present disclosure is to address the above-noted and other problems and to provide a method of manufacturing transition metal oxide fine particles having a size smaller than several micrometers, preferably smaller than micrometers (μm) to maintain a material property of a base material by a large surface area when coated on the base material having a relatively large surface area.

Another object of the present disclosure is to provide a method of manufacturing an additive configured to maintain dispersion stability even when the thickness of a coating layer collated on a base material is as thin as several hundred nanometers to several micrometers, and a method of manufacturing transition metal oxide fine particles that are included in the additive.

A further object of the present disclosure is to provide a method of producing transition metal oxide fine particles having a desired size by adjusting a reaction condition. In particular, the object of the present disclosure is to provide a process of preparing a strong base aqueous solution that is advantageous for atomization of a transition metal oxide, conditions for preparing the transition metal oxide, and the like.

A still further object of the present disclosure is to provide a method of producing transition metal oxide fine particles configured to prevent the formation of by-products during the producing process.

A still further object of the present disclosure is to provide a method of producing transition metal oxide fine particles that may solve a problem in that re-dispersion is difficult due to a strong secondary bonding.

A still further object of the present disclosure is to provide a method of producing transition metal oxide fine particles that may maintain stability even in contact with moisture.

Technical Solutions

To solve the above technical problems, a method for producing transition metal oxide fine particles may dissolve a transition metal oxide in a strong base aqueous solution, and precipitate transition metal oxide fine particles by titrating them in a strong base aqueous solution.

The method for producing the transition metal oxide fine particles may include heating a strong base aqueous solution while agitating the strong base aqueous solution; adding and dissolving a transition metal oxide in the heated strong base aqueous solution; re-dissolving solids generated in an interface between the strong base aqueous solution and a strong acid aqueous solution by adding and agitating the strong acid aqueous solution in the strong base aqueous solution in which the transition metal oxide is dissolved; precipitating transition metal oxide fine particles by adjusting pH of a mixed aqueous solution formed by mixing the strong base aqueous solution and the strong acid aqueous solution based on the addition speed and adding amount of the strong aqueous solution; and separating the transition metal oxide fine particles from the mixed aqueous solution, and sequentially washing, drying and heat-treating the separated transition metal oxide fine particles.

The strong base aqueous solution may be prepared by mixing water and sodium hydroxide (NaOH) in a mass ration of 6:1 to 10 to 1, and heating while agitating.

The purity of sodium hydroxide may be 99% or more.

In the process of heating the strong base aqueous solution while agitating it, the strong base aqueous solution may be heated at 60° C. to 105° C. while being agitated at 600 rpm to 700 rpm.

The transition metal oxide may include at least one selected from a group constituting of molybdenum trioxide and tungsten trioxide.

In the process of dissolving the transition metal oxide in the strong base aqueous solution, the molybdenum trioxide and the tungsten trioxide may be sequentially dissolved in the strong base aqueous solution.

The temperature when dissolving the tungsten trioxide in the strong base aqueous solution may be lower than the temperature when the molybdenum trioxide is dissolved in the strong base aqueous solution.

The purity of the molybdenum trioxide may be 99.5% or more and the purity of the tungsten trioxide is 99% or more.

In the process of dissolving the transition metal oxide in the strong base aqueous solution, the transition metal oxide may be dissolved to a saturated state.

In the process of dissolving the transition metal oxide in the strong base aqueous solution, the transition metal oxide may be agitated while being heated until solids of the strong base aqueous solution are completely dissolved

In the process of dissolving the transition metal oxide in the strong base aqueous solution, the transition metal oxide may be agitated while being heated, until the strong base aqueous solution becomes transparent.

In the process of dissolving the transition metal oxide in the strong base aqueous solution, the agitation conditions and the heating temperature conditions in the process of heating while agitating the strong base aqueous solution may be maintained.

The strong acid solution may be made of 60% to 75% nitric acid.

The process of re-dissolving the solids may include adding the strong acid aqueous solution; stopping the addition of the strong acid aqueous solution when the solids are generated; agitating the mixed aqueous solution of the strong base aqueous solution and the strong acid aqueous solution until the solids are re-dissolved in the mixed aqueous solution; and re-adding a strong acid aqueous solution when the solids are re-dissolved in the mixed aqueous solution.

In the process of precipitating the transition metal oxide fine particles, the transition metal oxide fine particles may be generated by adjusting pH of the mixed aqueous solution to be weakly basic.

In the process of precipitating the transition metal oxide fine particles, the transition metal oxide fine particles may be generated by lowering the pH of the mixed aqueous solution to 7.9 or 8.1.

In the process of separating the transition metal oxide fine particles from the mixed aqueous solution, the transition metal oxide fine particles may be filtered by a membrane, and in the process of washing the separated transition metal oxide fine particles, the transition metal oxide fine particles may be washed with distilled water of 100 g or less.

In the process of washing the separated transition metal oxide fine particles, the transition metal oxide fine particles washed with the distilled water may be additionally washed with an ethanol aqueous solution.

In the process of drying the transition metal oxide fine particles, freezing-drying for the transition metal oxide fine particles may be performed.

The process of drying the transition metal oxide fine particles may include primarily drying the transition metal oxide fine particles at room temperature for 24 hours or more; and secondarily drying the transition metal oxide fine particles in a vacuum while the temperature is raised to a temperature higher than room temperature.

In the process of heat-treating the transition metal oxide fine particles, the transition metal oxide fine particles separated from the mixed aqueous solution may be dried, and then heat-treated at a temperature of 380° C. to 450° C. for 30 minutes or more.

Advantageous Effect

The present disclosure may have following advantageous effects. The present disclosure may produce the transition metal oxide fine particles having the size of several micrometers to several hundred nanometers that is smaller than a micrometer. In particular, to suppress the solids generated during the process of producing the transition metal oxide fine particles, the present disclosure may include the agitating and heating the strong base aqueous solution, and dissolving the solids and the suspended matter generated in the process of producing the transition metal oxide fine particles, thereby producing the pure transition metal oxide fine particles.

Furthermore, the present disclosure may produce the strong base aqueous solution that is advantageous in generating the transition metal oxide fine particles by setting the conditions such as the mass ratio between water and sodium hydroxide, the agitation speed, the heating temperature and the like.

Still further, the present disclosure may provide an advantageous environment for atomization of the transition metal oxide by setting the conditions such as the order of adding the molybdenum trioxide and the tungsten trioxide to the strong base aqueous solution, the agitation speed, the heating temperature and the like.

Still further, the present disclosure may provide the conditions such as the washing, the drying, and the heat treatment to prevent insufficient crystallization of the transition metal oxide fine particles or the agglomeration of the transition metal oxide fine particles with each other, thereby producing the transition metal oxide fine particles having the desired nano-unit size.

Still further, the transition metal oxide fine particles produced according to the present disclosure may be used as a material constituting the coating solution to give antibacterial and catalytic properties to the base material when the coating layer is formed. In addition, the transition metal oxide fine particles may maintain the material properties of the base material due to its large surface area.

In particular, the transition metal oxide fine particles produced according to the present disclosure may be a stable component of a thin film coating layer with a thickness of several hundred nanometers.

It could be difficult to re-disperse the fine particles in the solvent constituting the collating solution due to the strong secondary bonding between the fine particles. However, the present disclosure may dry the transition metal oxide fine particles through the freezing-drying process, and then form the porous fine particles, thereby solving the difficulty of the re-dispersion.

The present disclosure may change the phase of the transition metal oxide fine particles to the alpha phase that is the stable phase through the heat treatment, thereby maintaining dispersion stability even the contact with moisture.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart showing a method of producing transition metal oxide fine particles according to a first embodiment of the present disclosure;

FIGS. 2 to 9 are schematic views showing the method of producing the transition metal oxide fine particles according to the first embodiment;

FIG. 10 is a flow chart showing a method of producing transition metal oxide fine particles according to a second embodiment of the present disclosure;

FIGS. 11 to 18 are schematic views showing the method of producing the transition metal oxide fine particles according to the second embodiment;

FIG. 19 is an electron micrograph of molybdenum trioxide powder, which is a raw material of the transition metal oxide fine particles according to the first embodiment;

FIG. 20 is an electron micrograph of tungsten trioxide powder, which is a raw material of the transition metal oxide fine particles according to the first embodiment;

FIG. 21 is an electron micrograph before heat treatment of molybdenum trioxide fine particles according to the first embodiment;

FIG. 22 is an electron micrograph after the heat treatment of molybdenum trioxide fine particles according to the first embodiment;

FIGS. 23 a to 23 d are electron micrographs of molybdenum/tungsten (Mo/W) mixed oxide crystal powder according to the first embodiment for each heat treatment temperature;

FIGS. 24 and 25 are electron micrographs of Mo/W mixed oxide crystal powder produced according to the first embodiment;

FIG. 26 is an electron micrograph of a coating surface to which the Mo/W mixed oxide crystal powder of FIG. 24 is added;

FIGS. 27 a to 27 c are electron micrographs of Mo/W mixed oxide crystal powder according to the second embodiment after heat treatment;

FIGS. 28 a and 28 b are graphs showing the component analysis results of the Mo/W mixed oxide crystal power produced according to the second embodiment;

FIGS. 28 c and 28 d are electron micrographs showing the component analysis results of the Mo/W mixed oxide crystal power produced according to the second embodiment;

FIG. 29 is a graph showing the results of measuring the diameter of the Mo/W mixed oxide crystal powder for each agitating speed and appropriate time;

FIGS. 30 to 31 are graphs showing the results of measuring the diameter of the Mo/W mixed oxide crystal powder for each reaction temperature;

FIG. 32 is an XRD graph of the Mo/W mixed oxide crystal powder for each reaction temperature;

FIG. 33 is an electron micrograph of the Mo/W mixed oxide crystal powder for each heat treatment temperature;

FIG. 34 is an XRD graph of the Mo/W mixed oxide crystal powder for each heat treatment temperature; and

FIG. 35 is a graph showing the results of measuring the diameter of the Mo/W mixed crystal powder before and after heating treatment in an optimized heat treatment method.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENT

Hereinafter, a method of producing transition metal oxide fine particles according to the present disclosure will be described in detail, referring to the accompanying drawings. Below, preferred embodiments according to the disclosure are specifically described with reference to the accompanying drawings. In the drawings, identical reference numerals can denote identical or similar components.

Embodiment 1

FIG. 1 is a flow chart showing a method of producing (or preparing) transition metal oxide fine particles according to Embodiment 1 (i.e., a first embodiment) of the present disclosure. FIGS. 2 to 9 are schematic views showing the method of producing the transition metal oxide fine particles according to the first embodiment.

As shown in FIG. 1 , to prepare the transition metal oxide fine particles, a strong base aqueous solution may be prepared, and the prepared strong base aqueous solution is heated while being agitated (S100).

Water (L) and sodium hydroxide (NaOH) may be mixed and heated while being agitated, thereby preparing the strong base aqueous solution. FIGS. 2 and 3 illustrate the process of preparing the strong base aqueous solution. In order to suppress a side reaction caused by foreign substances, it is preferred to use a sodium hydroxide having a purity of 99% or more.

To secure an appropriate reaction and sufficient solubility of the transition metal oxide in the subsequent process, the pH of the aqueous solution of a strong base should be set in an appropriate range. The pH of the aqueous solution of the strong base may be determined based on the ratio of water, and sodium hydroxide may be mixed in a mass ratio of 6:1 to 10:1. If the amount of sodium hydroxide is greater than 6:1 (e.g., the mass ratio of water and sodium hydroxide is 3:1), the basicity might be too strong and the reaction speed of the subsequent process might be excessively increased. Due to this, the time of particle nucleation might be also accelerated, and sufficient atomization of the transition metal oxide will not proceed. On the other hand, if the amount of sodium hydroxide is less than 10:1 (e.g., the mass ratio of water and sodium hydroxide is 15:1), sufficient basicity for dissolving the transition metal oxide could not be expressed.

The strong base aqueous solution may be heated to 60° C. to 105° C., while being agitated at 600 rpm to 700 rpm.

Agitation may be performed by an agitator that is rotatable at a rotation speed of 600 rpm to 700 rpm, and by ultrasonic waves providing the same effect as being agitated by the agitator. For example, the agitation may be performed in a bath sonicator for 5 minutes or more.

The reason for heating the strong base aqueous solution in the above range is to increase the solubility of the transition metal oxide in the subsequent process, and to suppress the formation of by-products. When the temperature of the strong base aqueous solution is lower than 60° C., the effect of improving the solubility of the transition metal oxide could be insignificant and the size of the synthesized particles could not be uniform. In addition, when the temperature of the strong base aqueous solution is lower than 60° C., sufficient energy required for dissolution of the transition metal oxide could not be supplied only to cause a decrease in the reaction speed. Conversely, when the temperature of the strong base aqueous solution is higher than 105° C., the reactor might damage.

Moreover, when the temperature of the strong base aqueous solution is out of the range of 60 to 105° C., by-products such as Na₂MoO₄, Na₂MoO₄·2H₂O, Na₂WO₄, and Na₂WO4·2H₂O might be generated. The by-products consume the raw material, thereby reducing the production amount of the desired transition metal oxide fine particles.

The agitating and heating of the strong base aqueous solution may be continued until the sodium hydroxide is completely dissolved in water and also continued in the subsequent process.

The strong base aqueous solution may be used as a solvent for dissolving the transition metal oxide. Transition metal oxides (e.g., molybdenum trioxide and/or tungsten trioxide) to be used as raw materials are not readily soluble in water but are readily soluble in basic aqueous solution. Unlike a dry environment using a ball mill, it is possible to control the particles of transition metal oxide in water environment using a strong aqueous solution.

Referring back to FIG. 1 , the transition metal oxide may be dissolved in a heated strong base aqueous solution (S200).

FIG. 4 shows a process of adding the transition metal oxide powder to the strong base aqueous solution. The transition metal oxide may include at least one selected from the group consisting of molybdenum trioxide and tungsten trioxide. For example, molybdenum trioxide or tungsten trioxide may be used along or they may be used together.

To secure the purity of the transition metal oxide fine particles, the purity of molybdenum trioxide may be 99.5% or more and the purity of tungsten trioxide may be 99% or more. Preferably, the purity of molybdenum trioxide and tungsten trioxide needs to be 99.9% or more.

When the transition metal oxide is added to the strong base aqueous solution, suspended matter may be formed. The suspended matter refers to the transition metal oxide powder that is not dissolved in the strong base aqueous solution. To sufficiently dissolve the transition metal oxide in the strong base aqueous solution, the suspended matter should be sufficiently agitated until it is dissolved completely. When they are ionized, the molybdenum trioxide and tungsten trioxide turn to colorless and transparent. Accordingly, whether all of the suspended matter has been dissolved may be known by whether the aqueous solution of the strong base aqueous solution, in which the transition metal oxide is dissolved, becomes colorless and transparent.

FIG. 5 shows a process of heating the strong base aqueous solution, while agitating it, to dissolve the suspended matter generated in the strong base aqueous solution. In the process of dissolving the transition metal oxide in the strong base aqueous solution, the agitating condition and the heated temperature condition in the process of heating the strong base aqueous solution while agitating it may be continuously maintained.

To improve the efficiency of the transition metal oxide fine particle preparation, the transition metal oxide may be dissolved in the strong base aqueous solution to a saturated state. The saturated state refers to a state in which the solute is dissolved in the solvent as much as possible so that it cannot be dissolved any more. If reaction proceeds with a saturated solution, the amount of transition metal oxide fine particles to be obtained in one reaction may be the maximum.

When the transition metal oxide includes both the molybdenum trioxide and the tungsten trioxide, it is preferable to sequentially dissolve the molybdenum trioxide and tungsten trioxide in the strong base aqueous solution rather than simultaneously dissolving them. For example, after the molybdenum trioxide is completely dissolved in the strong base aqueous solution to the saturated state, the tungsten trioxide may be dissolved in the strong base aqueous solution to the saturated state.

This is because sequential dissolution is advantageous for nucleation and atomization of transition metal oxides. If the molybdenum trioxide and the tungsten trioxide are simultaneously dissolved in the strong base aqueous solution, transition metal oxide having a fine particle size larger than a desired size could be generated.

In particular, the molybdenum trioxide dissolves better at high temperatures than the tungsten trioxide. Accordingly, the temperature at which the tungsten trioxide is dissolved in the strong base aqueous solution may be lower than the temperature at which the molybdenum trioxide is dissolved in it.

Referring back to FIG. 1 , next, a strong acid aqueous solution may be added to the strong base aqueous solution and agitated, to dissolve the solids generated at the interface again (S300). This process may be a process of precipitating the transition metal oxide fine particles by titrating the strong base aqueous solution containing the transition metal oxide with the strong acid aqueous solution.

The strong acid aqueous solution may include 60 to 74% of Nitric acid, and the user of other strong acid aqueous solutions is not excluded. However, when hydrochloric acid, formic acid and acetic acid are used, it may be difficult to produce uniform particles.

In an initial stage of adding the strong acid aqueous solution to the strong base aqueous solution, a strong neutralization reaction between the strong acid and the strong base may occur. At this time, solids may be rapidly generated on an interface between the strong base aqueous solution and the strong acid aqueous solution.

Particles of the solids rapidly generated at the interface between the strong base aqueous solution and the strong acid aqueous solution cannot be controlled as intended, thereby corresponding to by-products. Accordingly, in order to prepare monodisperse transition metal oxide fine particles, the mixed aqueous solution should be agitated to dissolve the solids inevitably generated in the mixed aqueous solution again. The mixed aqueous solution refers to a mixture of strong base aqueous solution and strong acid aqueous solution. FIG. 6 shows a process of re-dissolving the solids by adding and agitating the strong acid aqueous solution to the strong base aqueous solution.

To completely dissolve the solids in the mixed aqueous solution, the adding of the strong acid aqueous solution and the dissolving of the solids should be sequentially performed. When the solids are generated during the process of adding the strong acid aqueous solution, the strong acid aqueous solution adding may be stopped, and the mixed aqueous solution may be agitated until the solids are completely dissolved in the mixed aqueous solution. When the solids are completely re-dissolved, the strong acid aqueous solution may be re-added to repeat the above-described process.

Referring back to FIG. 1 , pH of the mixture may be adjusted by adjusting the adding speed and the adding amount of the strong acid aqueous solution, thereby precipitating the transition metal oxide fine particles (S400).

In order to suppress the generation of the by-products and prepare the transition metal oxide fine particles having a desired size, precipitation reaction should be induced to occur in the entire mixed aqueous solution, not at the interface between the strong base aqueous solution and the strong acid aqueous solution. To this end, the addition speed and the amount of the strong acid aqueous solution added to the strong base aqueous solution should be adjusted.

The addition speed of the strong acid aqueous solution may be related to the adjustment of the reaction speed for suppressing the generation of the solids described in S300. When the addition speed is adjusted by the method of adding the strong acid aqueous solution slowly, the drastic reaction between the strong base and the strong acid may be reduced enough to suppress the generation of solids, and the precipitation reaction may occur in the entire area of the mixed aqueous solution, not at only the interface between the strong base aqueous solution and the strong acid aqueous solution.

When the strong acid aqueous solution is added to the strong base aqueous solution, white particles may be generated. As the reaction is proceeding continuously, the mixed aqueous solution may turn to pale green. Hence, when the reaction is continued again, the mixed aqueous solution may turn to opaque white and the reaction may then finish. The mixed aqueous solution may become a suspension.

When the strong acid aqueous solution is added to the strong base aqueous solution, pH of the mixed aqueous solution may be gradually adjusted to be slightly basic. When the pH of the mixed aqueous solution is lowered to 7.9 to 8.1 by adjusting the added content of the strong acid aqueous solution, a change in solubility may occur so that transition metal oxide precipitation reaction may occur and transition metal oxide particles may be precipitated. FIG. 7 shows the transition metal oxide fine particles precipitated from the mixed aqueous solution.

Through the above-described process, the transition metal oxide fine particles may grow, and then the transition metal oxide fine particles having a predetermined size that cannot be achieved by the conventional physical grinding method. In addition, the size deviation of the transition metal oxide fine particles may have normal distribution.

Referring back to FIG. 1 , washing, drying and heat treatment processes may be sequentially performed by separating the transition metal oxide fine particles from the mixed aqueous solution (S500).

The method of separating the transition metal oxide fine particles from the mixed aqueous solution may include a process of filtering the transition metal oxide fine particles from the mixed aqueous solution by using a membrane filter or a process of separating the particles from the mixed aqueous solution by using centrifugation. A membrane filter having 200 nm pores may be used as the membrane filter.

Since the separated transition metal oxide fine particles tend to agglomerate with each other due to the salt contained in the mixed aqueous solution, the mixed aqueous solution on the surface of the transition metal oxide fine particles should be removed quickly. In particular, sodium (Na) contained in the transition metal oxide fine particles should be removed quickly, because it causes the formation of by-products (e.g., Na₂MoO₄, Na₂MoO₄·2H₂O, Na₂WO₄, Na₂WO₄·2H₂O).

The synthesized transition metal oxide fine particles may be washed with distilled water or deionized water. When the number of washing is increased, the dispersion stability of the transition metal oxide fine particles may be improved. The dispersion stability means that the transition metal oxide fine particles added to a solvent constituting a coating solution for applying them to a collating layer can maintain a dispersed state without agglomerating with each other in the solvent.

However, since the synthesized transition metal oxide fine particles may be easily soluble in the distilled water in an amorphous state, the amount of distilled water should be minimized when washed with the distilled water. For example, when using distilled water in an amount of exceeding 100 g, the yield may be reduced to 50% or less and it may be preferred to use distilled water of 100 g or less.

The transition metal oxide fine particles washed with the distilled water may be further washed with an ethanol aqueous solution. The synthesized transition metal oxide fine particles will not be dissolved in the ethanol aqueous solution. Accordingly, when the transition metal oxide fine particles are further washed in the ethanol aqueous solution, the drying time may be reduced in the subsequent drying process.

Next, the washed transition metal oxide fine particles may be dried.

When the transition metal oxide fine particles have a size of several hundred nanometers, secondary bonding (i.e., hydrogen bonding) between the fine particles may occur due to hydrogen reaction or Van des Waals force during the drying, and this might cause the fine particles to agglomerate with each other. In this instance, it may be difficult to re-disperse the fine particles having the above size in the solvent constituting the collating solution. In particular, when the tungsten trioxide is included as a raw material of the transition metal oxide fine particles, the size of the transition metal oxide fine particles may be adjusted to several hundred nanometers so that there may be a high possibility of the fine particles agglomeration.

To prevent that, freezing-drying may be used. Freezing-drying refers to a method of removing moisture from a target object by freezing the target object and sublimating ice, thereby obtaining a dried product. Once freezing the transition metal oxide fine particles and sublimating the ice, the dried transition metal oxide fine particles may be obtained. FIG. 8 shows the freezing-drying process.

The dry transition metal oxide fine particles gained by the freezing-drying method may have a porous structure and pores may exist where the ice is sublimated. Accordingly, the agglomeration between the fine particles may be suppressed by the pores.

In addition to the freezing-drying method, a room-temperature drying method and a vacuum-drying method, which are performed primarily and secondarily. A primary drying may be performed for 24 hours or more. In particular, when drying the washed transition metal oxide fine particles immediately at a temperature exceeding room temperature, the agglomeration between the fine particles might occur under the influence of moisture. Accordingly, the primary drying should be performed at room temperature, before the subsequent secondary drying. Once the primary drying is completed, the transition metal oxide fine particles having been dried primarily may be secondarily dried in a vacuum environment while raising the temperature to the temperature higher than room temperature. The vacuum drying may be performed in a vacuum open.

The dried transition metal oxide fine particles may be power that is still amorphous in a hydrate type. Accordingly, the dried transition metal oxide fine particles may have poor dispersion stability in the solvent constituting the coating solution. To maximize the dispersion stability of the transition metal oxide fine particles, a heat treatment process may be required and FIG. 9 shows a heat treatment process.

The heat treatment may be performed at a temperature of 380° C. to 450° C. for 30 minutes or more after drying the transition metal oxide fine particles. The heat treatment temperature and time may be conditions for changing the phase of the transition metal oxide fine particles to a crystalline state. Crystallization will not sufficiently occur at an heat treatment temperature lower than 380° C. Agglomeration between fine particles could occur at a heat treatment temperature higher than 450° C. The transition metal oxide fine particles for each heat treatment temperature will be described later, referring to FIGS. 14 a to 14 d.

When being heat-treated, the dried transition metal oxide fine particles may be converted from a hydrate state to a stable phase to have a characteristic of not eluting when in contact with moisture. The transition metal oxide fine particles prepared as described above may have dispersion stability in the solvent constituting the coating solution.

When molybdenum trioxide is used as the raw material of the transition metal oxide fine particles, porous fine particles with a level of several micrometers may be prepared. At this time, the level of several micrometers may be the size that is smaller than about 2 μm. When tungsten trioxide is used or both of the molybdenum trioxide and tungsten trioxide are used as the raw material of the transition metal oxide fine particles, the size of the transition metal oxide fine particles may be several hundred nanometers, preferably, about 700 nm.

The transition metal oxide fine particles prepared through the above process may have a large surface area due to the very small size, compared to the same mass. Accordingly, when added to the collating solution for forming a coating layer on a base material (e.g., a polymer), the transition metal oxide fine particles according to the present disclosure prepared through the above process may maintain dispersion stability and material properties of the base material.

When the raw material of the transition metal oxide includes both of the molybdenum trioxide and the tungsten trioxide, Mo—W—O molecular formula may be derived from X-ray fluorescence (XRF) analysis as shown in Table 1.

TABLE 1 Classification W Mo Na O wt. % 45.45 27.98 0.56 26.04 at. % 11.28 13.31 1.11 74.30

When the molecular formula of the transition metal oxide fine particles is Mo_(x)—W_(y)—O_(z), x, y, z may be calculated as follows:

x=13.31/((11.28+13.31)/2)=1.08(1.0825 . . . )

y=11.28/((11.28+13.31)/2)=0.92(0.9174 . . . )

z=(74.30−1.11*3)/((11.28+13.31)/2)=5.78(5.7722 . . . )

The molecular formula of the synthesized transition metal oxide fine particles may be Mo_(1.08)W_(0.92)O_(5.72)=Mo₁W_(0.85)O_(5.30).

Second Embodiment

FIG. 10 is a flow chart showing a method of producing transition metal oxide fine particles according to a second embodiment of the present disclosure. FIGS. 11 to 18 are schematic views showing the method of producing the transition metal oxide fine particles according to the second embodiment.

As shown in FIG. 10 , to prepare the transition metal oxide fine particles, first, sodium hydroxide (NaOH) and a surfactant may be added to water and then heated while agitated to form the strong base aqueous solution (S1000).

FIGS. 11 and 12 show the process of preparing the strong base aqueous solution. In order to suppress a side reaction caused by foreign substances, a sodium hydroxide having a plurality of 99% or more may be used.

In this instance, the surfactant may be added to reduce the particle size. Surfactants are the most representative amphiphilic compounds and have opposite functional groups of hydrophilics and hydrophobics in one molecule. Accordingly, surfactants are absorbed on the interface between liquid and liquid or between liquid and solid to cause various physical phenomena.

In the present disclosure, any surfactant may be used without limitations as long as a hydrophilic moiety and a hydrophobic moiety exist in one molecule. Specifically, the surfactant may include one or more selected from a cationic surfactant, an amphoteric surfactant, an anionic surfactant, a nonionic surfactant, etc. As a specific example, the surfactant may be Triton X-100.

The surfactant may be added in a weight ratio of 0.5 g or less per 1 L of water. When the surfactant is added in an amount exceeding 0.5 g per 1 L of water, agglomeration might occur due to agglomeration of particles in the subsequent process.

To secure an appropriate reaction and sufficient solubility of the transition metal oxide in the subsequent step, the pH of the strong base aqueous solution should be set in an appropriate range. The pH of the strong base aqueous solution may determined based on the ratio of water and sodium hydroxide. Water and sodium hydroxide may be mixed in a mass ratio of 6:1 to 10:1. If the amount of sodium hydroxide is greater than 6:1 (e.g., the mass ratio of water and sodium hydroxide is 3:1), the basicity might be too strong and the reaction speed in the subsequent process may be excessively increased. Accordingly, the time of particle nucleation could be also accelerated and sufficient atomization of the transition metal oxide could not proceed. Conversely, when the amount of sodium hydroxide is less than 10:1 (e.g., the mass ratio of water and sodium hydroxide is 15:1), sufficient basicity for dissolving the transition metal oxide could not be expressed.

While agitated at 600 rpm to 700 rpm, the strong base aqueous solution may be heated at 60° C. to 105° C.

The agitation may be performed by an agitator that is rotatable at a rotation rate of 600 rpm to 700 rpm, or by ultrasonic waves providing the same effect as the agitation of the agitator. For example, the agitation may be performed in a sonicator for 5 minutes or more.

The reason why the strong base aqueous solution is heated in the above range is to increase the solubility of the transition metal oxide in the subsequent process and to suppress the by-product formation. When the temperature of the strong base aqueous solution is less than 60° C., the effect of improving the solubility could be insignificant and the size of the synthesized particles could not be uniform. In addition, when the temperature of the strong base aqueous solution is less than 60° C., sufficient energy required for dissolution of the transition metal oxide cannot be supplied, thereby causing a decrease in the reaction speed. Conversely, when the temperature of the strong base aqueous solution exceeds 105° C., the reactor might be damaged.

Furthermore, when the temperature of the strong base aqueous solution is out of the range of 60° C. to 105° C., by-products such as Na₂MoO₄, Na₂MoO₄·2H₂O, Na₂WO₄, Na₂WO₄·2H₂O may be generated after adding the transition metal oxide. Such by-products consume the raw material, thereby reducing the production amount of the desired transition metal oxide fine particles.

The agitating and the heating of the strong base aqueous solution may be continued not only until the sodium hydrate is completely dissolved in the water but also in subsequent process.

The strong base aqueous solution may be used as the solvent for dissolving the transition metal oxide. The transition metal oxide to be used as the raw material such as the molybdenum trioxide and/or the tungsten trioxide is easily dissolved in water but not easily dissolved in basic aqueous solutions. Unlike the dry environment using the ball mill, in a wet environment using the strong base aqueous solution, it is possible to adjust the particles of the transition metal oxide.

Referring back to FIG. 10 , next, the transition metal oxide may be dissolved in the heated strong base aqueous solution (S2000).

FIG. 13 shows a process of adding the transition metal oxide powder to the strong base aqueous solution. The transition metal oxide may include at least one selected from the group of the molybdenum trioxide and the tungsten trioxide. For example, the molybdenum trioxide or the tungsten trioxide may be used alone, or they may be used together. Among the options, it may be preferred to use both of the molybdenum trioxide and the tungsten trioxide for the transition metal oxide, so as to produce fine particles.

The purity of the molybdenum trioxide may be 99.5% or more and the purity of the tungsten trioxide may be 99% or more, in order to secure the purity of the transition metal oxide fine particles. Preferably, the purity of the molybdenum trioxide and that of the tungsten trioxide may be 99.9% or more.

When the transition metal oxide is added to the strong base aqueous solution, suspended matter could be generated. The suspended matter refers to transition metal oxide powder that fails to be dissolved in the strong base aqueous solution. To dissolve the transition metal oxide in the strong base aqueous solution, agitation should be sufficiently performed until all of the suspended matter is dissolved in the aqueous solution. When they are ionized, the molybdenum trioxide and the tungsten trioxide turn colorless and transparent. Accordingly, whether all of the suspended matter has been dissolved may be known by whether the strong base aqueous solution containing the dissolved transition metal oxide becomes transparent.

FIG. 14 shows the process of heating the strong base aqueous solution while agitating it in order to dissolve the suspended matter (or solids) formed in the strong base aqueous solution.

In this process, the strong base aqueous solution may be agitated at a speed of 100 rpm to 300 rpm while being heated at 50° C. to 100° C. until the strong base aqueous solution becomes transparent.

The agitation may be performed by an agitator that is rotatable at a rotation speed of 100 rpm to 300 rpm, and by ultrasonic waves providing the same effect as being agitated by the agitator. For example, the agitation may be performed in a bath sonicator for 5 minutes or more.

When the temperature of the strong base aqueous solution is lower than 50° C. or the agitation speed is less than 100 rpm, the effect of improving the solubility of the transition metal oxide could be insignificant and the size of the synthesized particles could not be uniform. Conversely, when the temperature of the strong base aqueous solution is higher than 100° C. or the agitation speed exceeds 300 rpm, agglomeration could be caused by collision between the transition metal oxide powders, thereby making it difficult to prepare the fine particles.

To improve the efficiency of the transition metal oxide fine particle preparation, the transition metal oxide may be dissolved in the strong base aqueous solution to a saturated state. The saturated state refers to a state in which the solute is dissolved in the solvent as much as possible so that it cannot be dissolved any more. If reaction proceeds with a saturated solution, the amount of transition metal oxide fine particles to be obtained in one reaction may be the maximum.

When the transition metal oxide includes both the molybdenum trioxide and the tungsten trioxide, it may be preferred to dissolve the molybdenum trioxide and the tungsten trioxide in the strong base aqueous solution sequentially than simultaneously. For example, after the molybdenum is completely dissolved in the strong base aqueous solution to a saturated state, the tungsten trioxide may be dissolved therein to a saturated state.

This is because sequential dissolution is advantageous for nucleation and atomization of transition metal oxides. If the molybdenum trioxide and the tungsten trioxide are simultaneously dissolved in the strong base aqueous solution, transition metal oxide having a fine particle size larger than a desired size could be generated.

In particular, the molybdenum trioxide dissolves better at high temperatures than the tungsten trioxide. Accordingly, the temperature at which the tungsten trioxide is dissolved in the strong base aqueous solution may be lower than the temperature at which the molybdenum trioxide is dissolved in it

Referring back to FIG. 10 , next, a strong acid aqueous solution may be added to the strong base aqueous solution and agitated, to dissolve the solids generated at the interface again (S3000). This process may be a process of precipitating the transition metal oxide fine particles by titrating the strong base aqueous solution containing the transition metal oxide with the strong acid aqueous solution.

The strong acid aqueous solution may include 60 to 74% of Nitric acid, and the user of other strong acid aqueous solutions is not excluded. However, when hydrochloric acid, formic acid and acetic acid are used, it may be difficult to produce uniform particles.

In an initial stage of adding the strong acid aqueous solution to the strong base aqueous solution, a strong neutralization reaction between the strong acid and the strong base may occur. At this time, solids may be rapidly generated on an interface between the strong base aqueous solution and the strong acid aqueous solution.

Particles of the solids rapidly generated at the interface between the strong base aqueous solution and the strong acid aqueous solution cannot be controlled as intended, thereby corresponding to by-products. Accordingly, in order to prepare monodisperse transition metal oxide fine particles, the mixed aqueous solution should be agitated to dissolve the solids inevitably generated in the mixed aqueous solution again. The mixed aqueous solution refers to a mixture of strong base aqueous solution and strong acid aqueous solution. FIG. 15 shows a process of re-dissolving the solids by adding and agitating the strong acid aqueous solution to the strong base aqueous solution.

To completely dissolve the solids in the mixed aqueous solution, the adding of the strong acid aqueous solution and the dissolving of the solids should be sequentially performed. When the solids are generated during the process of adding the strong acid aqueous solution, the strong acid aqueous solution adding may be stopped, and the mixed aqueous solution may be agitated until the solids are completely dissolved in the mixed aqueous solution. When the solids are completely re-dissolved, the strong acid aqueous solution may be re-added to repeat the above-described process.

Referring back to FIG. 10 , pH of the mixture may be adjusted by adjusting the adding speed and the adding amount of the strong acid aqueous solution, thereby precipitating the transition metal oxide fine particles (S4000).

In order to suppress the generation of the by-products and prepare the transition metal oxide fine particles having a desired size, precipitation reaction should be induced to occur in the entire mixed aqueous solution, not at the interface between the strong base aqueous solution and the strong acid aqueous solution. To this end, the addition speed and the amount of the strong acid aqueous solution added to the strong base aqueous solution should be adjusted.

The addition speed of the strong acid aqueous solution may be related to the adjustment of the reaction speed for suppressing the generation of the solids described in S3000. When the addition speed is adjusted by the method of adding the strong acid aqueous solution slowly, the drastic reaction between the strong base and the strong acid may be reduced enough to suppress the generation of solids, and the precipitation reaction may occur in the entire area of the mixed aqueous solution, not at only the interface between the strong base aqueous solution and the strong acid aqueous solution. Accordingly, the strong acid aqueous solution may be titrated within 10 minutes, because the nucleation time should be shortened and the particle size can be reduced, after the strong acid aqueous solution is shortened within 10 minutes.

When the strong acid aqueous solution is added to the strong base aqueous solution, white particles may be generated. As the reaction is proceeding continuously, the mixed aqueous solution may turn to pale green. Hence, when the reaction is continued again, the mixed aqueous solution may turn to opaque white and the reaction may then finish. The mixed aqueous solution may become a suspension.

When the strong acid aqueous solution is added to the strong base aqueous solution, pH of the mixed aqueous solution may be gradually adjusted to be slightly basic. When the pH of the mixed aqueous solution is lowered to 7.9 to 8.1 by adjusting the added content of the strong acid aqueous solution, a change in solubility may occur so that transition metal oxide precipitation reaction may occur and transition metal oxide particles may be precipitated. FIG. 16 shows the transition metal oxide fine particles precipitated from the mixed aqueous solution.

Through the above-described process, the transition metal oxide fine particles may grow, and then the transition metal oxide fine particles having a predetermined size that cannot be achieved by the conventional physical grinding method. In addition, the size deviation of the transition metal oxide fine particles may have normal distribution.

Referring back to FIG. 10 , washing, drying and heat treatment processes may be sequentially performed by separating the transition metal oxide fine particles from the mixed aqueous solution (S5000).

The method of separating the transition metal oxide fine particles from the mixed aqueous solution may include a process of filtering the transition metal oxide fine particles from the mixed aqueous solution by using a membrane filter or a process of separating the particles from the mixed aqueous solution by using centrifugation. A membrane filter having 200 nm pores may be used as the membrane filter.

Since the separated transition metal oxide fine particles tend to agglomerate with each other due to the salt contained in the mixed aqueous solution, the mixed aqueous solution on the surface of the transition metal oxide fine particles should be removed quickly. In particular, sodium (Na) contained in the transition metal oxide fine particles should be removed quickly, because it causes the formation of by-products (e.g., Na₂MoO₄, Na₂MoO₄·2H₂O, Na₂WO₄, Na₂WO₄·2H₂O).

The synthesized transition metal oxide fine particles may be washed with distilled water or deionized water. When the number of washing is increased, the dispersion stability of the transition metal oxide fine particles may be improved. The dispersion stability means that the transition metal oxide fine particles added to a solvent constituting a coating solution for applying them to a collating layer can maintain a dispersed state without agglomerating with each other in the solvent.

However, since the synthesized transition metal oxide fine particles may be easily soluble in the distilled water in an amorphous state, the amount of distilled water should be minimized when washed with the distilled water. For example, when using distilled water in an amount of exceeding 100 g, the yield may be reduced to 50% or less and it may be preferred to use distilled water of 100 g or less.

The transition metal oxide fine particles washed with the distilled water may be further washed with an ethanol aqueous solution. The synthesized transition metal oxide fine particles will not be dissolved in the ethanol aqueous solution. Accordingly, when the transition metal oxide fine particles are further washed in the ethanol aqueous solution, the drying time may be reduced in the subsequent drying process.

Next, the washed transition metal oxide fine particles may be dried.

When the transition metal oxide fine particles have a size of several hundred nanometers, secondary bonding (i.e., hydrogen bonding) between the fine particles may occur due to hydrogen reaction or Van des Waals force during the drying, and this might cause the fine particles to agglomerate with each other. In this instance, it may be difficult to re-disperse the fine particles having the above size in the solvent constituting the collating solution. In particular, when the tungsten trioxide is included as a raw material of the transition metal oxide fine particles, the size of the transition metal oxide fine particles may be adjusted to several hundred nanometers so that there may be a high possibility of the fine particles agglomeration.

To prevent that, freezing-drying may be used. Freezing-drying refers to a method of removing moisture from a target object by freezing the target object and sublimating ice, thereby obtaining a dried product. Once freezing the transition metal oxide fine particles and sublimating the ice, the dried transition metal oxide fine particles may be obtained. FIG. 8 shows the freezing-drying process.

The dry transition metal oxide fine particles gained by the freezing-drying method may have a porous structure and pores may exist where the ice is sublimated. Accordingly, the agglomeration between the fine particles may be suppressed by the pores.

In addition to the freezing-drying method, a room-temperature drying method and a vacuum-drying method, which are performed primarily and secondarily. A primary drying may be performed for 24 hours or more. In particular, when drying the washed transition metal oxide fine particles immediately at a temperature exceeding room temperature, the agglomeration between the fine particles might occur under the influence of moisture. Accordingly, the primary drying should be performed at room temperature, before the subsequent secondary drying. Once the primary drying is completed, the transition metal oxide fine particles having been dried primarily may be secondarily dried in a vacuum environment while raising the temperature to the temperature higher than room temperature. The vacuum drying may be performed in a vacuum oven.

The dried transition metal oxide fine particles may be power that is still amorphous in a hydrate type. Accordingly, the dried transition metal oxide fine particles may have poor dispersion stability in the solvent constituting the coating solution. To maximize the dispersion stability of the transition metal oxide fine particles, a heat treatment process may be required and FIG. 18 shows a heat treatment process.

The heat treatment may be performed at a temperature of 380° C. to 480° C. for 30 minutes or more after drying the transition metal oxide fine particles. The heat treatment temperature and time may be conditions for changing the phase of the transition metal oxide fine particles to a crystalline state. Crystallization will not sufficiently occur at an heat treatment temperature lower than 380° C. Agglomeration between fine particles could occur at a heat treatment temperature higher than 480° C.

When being heat-treated, the dried transition metal oxide fine particles may be converted from a hydrate state to a stable phase to have a characteristic of not eluting when in contact with moisture. The transition metal oxide fine particles prepared as described above may have dispersion stability in the solvent constituting the coating solution.

When molybdenum trioxide is used as the raw material of the transition metal oxide fine particles, porous fine particles with a level of several micrometers may be prepared. At this time, the level of several micrometers may be the size that is smaller than about 2 μm.

When tungsten trioxide is used or both of the molybdenum trioxide and tungsten trioxide are used as the raw material of the transition metal oxide fine particles, the size of the transition metal oxide fine particles may be several hundred nanometers. It is checked that the size of the fine particles is 500 nm to 650 nm.

The transition metal oxide fine particles prepared through the above process may have a large surface area due to the very small size, compared to the same mass. Accordingly, when added to the collating solution for forming a coating layer on a base material (e.g., a polymer), the transition metal oxide fine particles according to the present disclosure prepared through the above process may maintain dispersion stability and material properties of the base material.

EMBODIMENTS

Hereinafter, the configuration and operation of the present disclosure will be described in detail through embodiments. However, the embodiments are described above with reference to a number of illustrative embodiments thereof. However, the present disclosure is not intended to limit the embodiments and drawings set forth herein, and numerous other modifications and embodiments can be devised by one skilled in the art.

Further, the effects and predictable effects based on the configurations in the disclosure are to be included within the range of the disclosure though not explicitly described in the description of the embodiments.

Embodiment 1

A method of producing transition metal oxide fine particles according to the embodiment refers to FIGS. 2 to 9 .

High-purity molybdenum trioxide with a purity of 99.95%, high-purity sodium hydroxide with a purity of 99% or more, and 70% nitric acid are used as materials for preparing the transition metal oxide fine particles.

To prepare the strong base aqueous solution, sodium hydroxide is mixed in a mass ratio of 3:1, and agitated with a bath sonicator for 5 minutes or more (see FIG. 2 ). Hence, the mixed strong base aqueous solution is heated to the temperature range of 80° C. to 100° C. (see FIG. 3 ).

Next, molybdenum trioxide is added to the strong base aqueous solution to a saturated state (see FIG. 4 ). Agitation is performed until suspended matter of the molybdenum trioxide is dissolved in the strong base aqueous solution (see FIG. 5 ).

Next, 70% nitric acid is slowly added to the strong base aqueous solution, and the solids generated in the interface between the strong base aqueous solution and 70% nitric acid are re-dissolved in the mixed aqueous solution by the agitation (see FIG. 6 ).

As 70% nitric acid is added to the strong base aqueous solution, pH of the mixed aqueous solution (NaOH₃) is lowered. When the pH of the mixed aqueous solution reaches about 8, crystal nucleation is generated and molybdenum trioxide fine particles are precipitated in the mixed aqueous solution (see FIG. 7 ).

The molybdenum trioxide fine particles are separated from the mixed aqueous solution and washed in deionized water, and heat-treated at 400° C. for 1 hour after freezing-drying the washed fine particles (see FIG. 9 ).

As a result, fine particles having a size of 2 μm or less that may maintain dispersion stability even in a wet environment where moisture exists are prepared.

In the above embodiment, a high-sensitivity pH meter electrode (Horiba F-72, 9618S-10D electrode) for measuring the pH of deionized water is used to measure the pH of the aqueous solution.

Next, the effect of the present disclosure will be verified by comparing the particle size before and after the agitation of the transition metal oxide fine particles.

FIG. 19 is an electron micrograph of molybdenum trioxide powder, which is a raw material of the transition metal oxide fine particles according to the first embodiment. FIG. 20 is an electron micrograph of tungsten trioxide powder, which is a raw material of the transition metal oxide fine particles according to the first embodiment.

The molybdenum trioxide powder and the tungsten trioxide powder prepared as the raw material may have a particle size that is tens of micrometers to hundreds of micrometers.

FIG. 21 is an electron micrograph before heat treatment of molybdenum trioxide fine particles according to the first embodiment. FIG. 22 is an electron micrograph after the heat treatment of molybdenum trioxide fine particles according to the first embodiment.

The molybdenum fine particles prepared through the processes before the heat treatment may have a hexagonal phase in a hydrate type, but the molybdenum fine particles after the heat treatment performed at 400° C. for 1 hour may have an alpha phase. The alpha phase is a stable phase with the property configured not to elute even in contact with water.

The molybdenum fine particles prepared according to the present disclosure may have a porous structure of 2 μm or less. The molybdenum fine particles may not agglomerate with each other even after general drying without the freezing-drying, and may be used as an additive in the coating solution in a stable state after heat treatment.

FIGS. 23 a to 23 d are electron micrographs of molybdenum/tungsten (Mo/W) mixed oxide crystal powder according to the first embodiment for each heat treatment temperature.

As a result of heat treatment at 350° C., crystallization did not occur sufficiently. As a result of heat treatment of 500° C., agglomeration between the fine particles occurred. As a result of heat treatment at 400° C. and 450° C., the phase of the fine particles was changed and became a crystallization state.

FIGS. 24 and 25 are electron micrographs of Mo/W mixed oxide crystal powder produced according to the first embodiment. FIG. 26 is an electron micrograph of a coating surface to which the Mo/W mixed oxide crystal powder of FIG. 24 is added.

The mixed oxide fine particles may have a size smaller than the fine particles shown in the electron micrograph of FIG. 22 . Due to this, there is possibility that agglomeration might occur, and a freezing-drying method may be preferred to prevent the agglomeration.

In the solvent constituting the coating solution, dispersion stability of the additive is required. In particular, a thin film coating layer may require that the transition metal oxide fine particles have a much smaller size to become a stable component of the thin film coating layer. As confirmed in the electron micrographs of FIGS. 24 and 25 , the mixed transition metal oxide fine particles may be a stable component of the thin film coating layer, because they have a size of several hundred nanometers. That may be checked in the electron micrograph of FIG. 26 .

For the electron micrographs of the present disclosure, morphology analysis of the sample surface was performed using scanning electron microscopy (SEM). In addition, elemental analysis was performed using an energy dispersive x-ray spectroscope (EDX), and particle size was analyzed using a particle size analyzer (e.g., PSA, 53500, Microtrac). The synthesized transition metal oxide fine particles are crystallized in a kiln, and the crystallized particles were analyzed using XRD (x-ray diffraction, D8 Advance, Bruker).

Embodiment 2

Embodiment 2 (or the second embodiment) of the method of producing the transition metal oxide fine particles may refer to FIGS. 10 to 18 .

The components for preparing the transition metal oxide fine particles may include 99.95% high purity molybdenum trioxide, 99.5% high purity tungsten trioxide, high purity sodium hydroxide over 99% purity, Trition X-100 as surfactant, and 65% nitric acid (HNO₃).

To produce the strong base aqueous solution, sodium hydroxide and water may be mixed in a mass ratio of 3:1, and 0.15 g, 1.0 g and 2.0 g per 1 L of water may be respectively mixed. After that, the mixed solution may be agitated in a bath sonicator for 5 minutes or more (see FIG. 11 ). Hence, the mixed strong base aqueous solution may be heated to a temperature range of 80° C. to 100° C. (see FIG. 12 ).

Next, the molybdenum trioxide and the tungsten trioxide may be added to the strong base aqueous solution to a saturated state (see FIG. 13 ). After that, the mixed solution is sufficiently agitated at 200 rpm in a temperature range of 60° C. to 105° C., until the molybdenum trioxide and suspended matter in the molybdenum trioxide are dissolved in the strong base aqueous solution (see FIG. 14 ).

Next, 65% nitric acid may be slowly added to the strong base aqueous solution, and solids generated in the interface between the strong base aqueous solution and the 65% nitric acid may be re-dissolved in the mixed aqueous solution during the agitation (see FIG. 15 ).

As the 65% nitric acid is added to the strong base aqueous solution, pH of the mixed aqueous solution (NaOH₃) may be lowered. When the pH of the mixed aqueous solution reaches near 8, crystalline nucleation may be generated in the entire area of the mixed aqueous solution, and the molybdenum/tungsten mixed oxide fine particles (MoWo₆) may be precipitated (see FIG. 16 ).

The molybdenum/tungsten mixed trioxide fine particles are separated from the mixed aqueous solution and freezing-drying is performed (see FIG. 17 ). After that, the fine particles are heat-treated for 60 to 105 minutes in a temperature range of 350° C. to 500° C. raised for the heat treatment at 5 to ° C./min (see FIG. 18 ).

As a result, fine particles having a size of 1 μm or less that may maintain dispersion stability even in a wet environment where moisture exists are prepared.

In this embodiment, a high-sensitivity pH meter electrode (Horiba F-72, 9618S-10D electrode) for measuring the pH of deionized water is used to measure the pH of the aqueous solution.

Next, the effect of the present disclosure will be verified by comparing the particle size before and after the agitation of the transition metal oxide fine particles.

FIGS. 27 a to 27 c are electron micrographs of Mo/W mixed oxide crystal powder according to the second embodiment after heat treatment. As shown in FIGS. 27 a to 27 c , Trition X-100 is added in an amount of 0.15 g, 1.0 g, and 2.0 g per 1 L of water, respectively, and agitated at a reaction temperature of 60° C. at 200 rpm. After that, HnO₃ is titrated for 5 minutes and heat treatment is performed for 1 hour at a temperature of 450° C. that is raised at a speed of 5° C./min.

As shown in FIG. 27 a , when the when 0.15 g of surfactant is added per 1 L of water, it is confirmed that molybdenum/tungsten mixed oxide fine particles (MoWO₆) having a porous structure of 2 μm or less are formed without agglomeration with each other.

On the other hand, as shown in FIGS. 27 b and 27 c , when 1.0 g and 2.0 g of the surfactant, which exceeds CMC, is added per 1 L of water, it is confirmed that there is agglomeration between the fine particles.

FIGS. 28 a and 28 b are graphs showing the component analysis results of the Mo/W mixed oxide crystal power produced according to the second embodiment. FIGS. 28 c and 28 d are electron micrographs showing the component analysis results of the Mo/W mixed oxide crystal power produced according to the second embodiment. At this time, in FIGS. 28 a to 28 d, 0.15 g of Trition X-100 is added per 1 L of water and agitated at a reaction temperature of 60° C. at a speed of 200 rpm. After that, NHO₃ is titrated for 5 minutes and heat treatment is performed for 1 hour at a raised temperature of 450° C.

s shown in FIGS. 28 a and 28 b , the molybdenum/tungsten mixed oxide fine particles prepared according to the second embodiment may have a reaction shown in Chemical Reaction Formula 1 below:

Reaction Formula:

WO₄ ²⁻+MoO₄ ²−+4H⁺→MoWO₆+2H₂O

The molybdenum/tungsten mixed oxide fine particles prepared according to the second embodiment may include MoWO6 having a structure of Chemical Formula below:

As shown in FIGS. 28 c and 28 d , it is confirmed based on the result of the component analysis for the molybdenum/tungsten mixed oxide fine particles (MoWO₆) prepared according to the second embodiment that the weight ratio is Mo:W:O=17.54% wt: 34.44% wt: 48.03% wt.

It is also confirmed that the molybdenum/tungsten mixed oxide fine particles (MoWO₆) prepared according to the second embodiment has a composite structure n which the polycrystalline structure of WO₃ and the orthohombic crystal structure of MoO₃ coexist.

FIG. 29 is a graph showing the results of measuring the diameter of the Mo/W mixed oxide crystal powder for each agitating speed and appropriate time. In FIG. 29 , Trition X-100 is added in an amount of 0.15 g per 1 L of water and agitated at a reaction temperature of 90° C. at a speed of 200 rpm to 900 rpm, and HNO₃ is titrated for 5 to 180 minutes. Also, heat treatment is performed for 1 hour while the temperature has been raised to 450° C. at a rate of 10° C./min.

As shown in FIG. 29 , as the agitation speed increases, agglomeration occurs due to collision between particles, thus the diameter of the molybdenum/tungsten mixed oxide fine particle tends to increase.

It is also confirmed that the shorter the titration time of HNO₃ is, the shorter the nucleation time is, so that the diameter of the molybdenum/tungsten mixed oxide fine particle may tend to decrease.

Accordingly, it is figured out that the agitation speed is lowered to 100 rpm to 300 rpm and the agitation is performed within 10 minutes of the HNO₃ titration time, which is advantageous to produce the fine particles.

FIGS. 30 to 31 are graphs showing the results of measuring the diameter of the Mo/W mixed oxide crystal powder for each reaction temperature. FIG. 32 is an XRD graph of the Mo/W mixed oxide crystal powder for each reaction temperature. In FIGS. 30 to 32 , Trition X-100 is added in an amount of 0.15 g per 1 L of water and agitated at reaction temperatures of 60° C., 80° C., 90° C. and 105° C. at a speed of 200 to 900 rpm, respectively, and HNO₃ is titrated for 5 minutes. Also, heat treatment is performed for 1 hour while the temperature has been raised to 450° C. at a rate of 10° C./min.

As shown in FIGS. 30 to 32 , the diameter of the molybdenum/tungsten mixed oxide fine particles (MoWO₆) for each reaction temperature is measured. As the result of the measurement, it is confirmed that fine particles having a diameter of approximately 610 nm or less are produced at reaction temperatures of 60° C., 80° C. and 90° C.

In addition, as a result of confirming the crystal structure of the molybdenum/tungsten mixed oxide fine particles (MoWO₆) for each reaction temperature, a crystal peak is shown at a reaction temperature of 60° C. or higher. At this time, Mo tends to precipitate at a high temperature and W tends to precipitate at a low temperature.

Accordingly, it is confirmed that the crystal structure of the molybdenum/tungsten mixed oxide fine particles (MoWO₆) is a composite structure in which the polycrystalline structure of WO₃ and the orthohombic crystal structure of MoO₃ coexist.

FIG. 33 is an electron micrograph of the Mo/W mixed oxide crystal powder for each heat treatment temperature. FIG. 34 is an XRD graph of the Mo/W mixed oxide crystal powder for each heat treatment temperature. In FIGS. 30 to 32 , Trition X-100 is added in an amount of 0.15 g per 1 L of water and agitated at a reaction temperature of 60° C. at a speed of 200 rpm, and HNO₃ is titrated for 5 minutes. Also, heat treatment is performed for 1 hour while the temperature has been raised to 350° C., 400° C., 450° C. and 500° C. at a rate of 10° C./min, respectively.

As shown in FIGS. 33 to 34 , it is confirmed based on the result of observing the molybdenum/tungsten mixed oxide fine particles for each heat treatment temperature that the oxide fine particles are in an amorphous state at 350° C. and that the alpha phase is formed at 400° C., 450° C. and 500° C.

FIG. 35 is a graph showing the results of measuring the diameter of the Mo/W mixed crystal powder before and after heating treatment in an optimized heat treatment method. In FIG. 35 , Trition X-100 is added in an amount of 0.15 g per 1 L of water and agitated at a reaction temperature of 60° C. at a speed of 200 rpm, and HNO₃ is titrated for 5 minutes. Also, heat treatment is performed for 1 hour while the temperature has been raised to 450° C. at a rate of 5° C./min, respectively.

As shown in FIG. 35 , when the heat treatment is performed for 1 hour in a state where the temperature is raised to 450° C. at the speed of 5° C./min, it is confirmed that the diameter of the molybdenum/tungsten mixed oxide fine particles before and after the heat treatment is measured the most finely (minutely).

The embodiments are described above with reference to a number of illustrative embodiments thereof. However, the present disclosure is not intended to limit the embodiments and drawings set forth herein, and numerous other modifications and embodiments can be devised by one skilled in the art. Further, the effects and predictable effects based on the configurations in the disclosure are to be included within the range of the disclosure though not explicitly described in the description of the embodiments. 

1. A method of producing transition metal oxide fine particles comprising: heating and agitating a strong base aqueous solution; adding and dissolving a transition metal oxide into the heated strong base aqueous solution; forming a mixed aqueous solution by a strong acid aqueous solution to the strong base aqueous solution in which the transition metal oxide is dissolved, and by agitating and dissolving solids at an interface between the strong base aqueous solution and the strong acid aqueous solution; controlling a speed or an amount of the strong aqueous solution to be added to the strong base aqueous solution, to adjust pH of the mixed aqueous solution and to precipitate transition metal oxide fine particles; and separating the transition metal oxide fine particles from the mixed aqueous solution, and washing, drying and heat-treating the separated transition metal oxide fine particles.
 2. The method of producing the transition metal oxide fine particles of claim 1, wherein the strong base aqueous solution is prepared by mixing water and sodium hydroxide (NaOH) having a mass ratio within a range of 6:1 to 10 to 1, and sodium hydroxide mixed with the water has a purity of 99% or more.
 3. The method of producing the transition metal oxide fine particles of claim 1, wherein the heating and the agitating of the strong base aqueous solution includes heating the strong base aqueous solution to be within a range of 60° C. to 105° C. and agitating the strong base aqueous solution within a range of 600 rpm to 700 rpm.
 4. The method of producing the transition metal oxide fine particles of claim 1, wherein the transition metal oxide comprises at least one selected from a group constituting of molybdenum trioxide and tungsten trioxide, and the dissolving of the transition metal oxide into the strong base aqueous solution includes sequentially dissolving the molybdenum trioxide and the tungsten trioxide into the strong base aqueous solution.
 5. The method of producing the transition metal oxide fine particles of claim 4, wherein a temperature when dissolving the tungsten trioxide into the strong base aqueous solution is less than a temperature when dissolving the molybdenum trioxide into the strong base aqueous solution, and the molybdenum trioxide dissolved into the strong base aqueous solution has a purity of 99.5% or more and the tungsten trioxide dissolved into the strong base aqueous solution has a purity of 99% or more.
 6. The method of producing the transition metal oxide fine particles of claim 1, wherein the dissolving of the transition metal oxide into the strong base aqueous solution; includes dissolving the transition metal oxide into a saturated state, or agitating the transition metal oxide while being heated until the solids of the strong base aqueous solution are completely dissolved, or agitating the transition metal oxide while being heated until the strong base aqueous solution becomes transparent.
 7. The method of producing the transition metal oxide fine particles of claim 1, wherein the dissolving of the transition metal oxide into the strong base aqueous solution includes maintaining agitation conditions and heating temperature conditions during the heating and the agitating of the strong base aqueous solution.
 8. The method of producing the transition metal oxide fine particles of claim 1, wherein the forming the mixed aqueous solution includes: adding the strong acid aqueous solution to the strong base aqueous solution; stopping the adding of the strong acid aqueous solution to the strong base aqueous solution based on the solids being generated at the interface; agitating the mixed aqueous solution until the solids are dissolved into the mixed aqueous solution; and again adding a strong acid aqueous solution to the strong base aqueous solution when the solids are dissolved into the mixed aqueous solution, and wherein the strong acid solution comprises 60% to 75% nitric acid.
 9. The method of producing the transition metal oxide fine particles of claim 1, wherein the precipitating of the transition metal oxide fine particles includes generating the transition metal oxide fine particles by adjusting pH of the mixed aqueous solution, or generating the transition metal oxide fine particles by lowering the pH of the mixed aqueous solution to be within a range of 7.9 or 8.1.
 10. The method of producing the transition metal oxide fine particles of claim 1, wherein the separating of the transition metal oxide fine particles from the mixed aqueous solution includes filtering the transition metal oxide fine particles by a membrane, the washing of the separated transition metal oxide fine particles; includes washing the transition metal oxide fine particles with distilled water of 100 g or less, and additionally washing the transition metal oxide fine particles with an ethanol aqueous solution.
 11. The method of producing the transition metal oxide fine particles of claim 1, wherein the drying of the transition metal oxide fine particles includes freezing-drying of the transition metal oxide fine particles.
 12. The method of producing the transition metal oxide fine particles of claim 1, wherein the drying of the transition metal oxide fine particles includes: primarily drying the transition metal oxide fine particles at room temperature for at least 24 hours; and secondarily drying the transition metal oxide fine particles in a vacuum environment at a temperature higher than the room temperature.
 13. The method of producing the transition metal oxide fine particles of claim 1, wherein the heat-treating of the transition metal oxide fine particles; includes drying the transition metal oxide fine particles separated from the mixed aqueous solution, and then heat treating the transition metal oxide fine particles at a temperature within a range of 380° C. to 450° C. for at least 30 minutes.
 14. A method of producing transition metal oxide fine particles comprising: providing a strong base aqueous solution by heating and agitating after adding a sodium hydroxide and a surfactant to water; adding a transition metal oxide to the heated strong base aqueous solution, and dissolving the transition metal oxide into the strong base aqueous solution; forming a mixed aqueous solution by adding a strong acid aqueous solution to the strong base aqueous solution in which the transition metal oxide is dissolved, and by agitating and dissolving solids at an interface between the strong base aqueous solution and the strong acid aqueous solution; controlling a speed or an amount of the strong aqueous solution to be added to the strong base aqueous solution, to adjust pH of the mixed aqueous solution and to precipitate transition metal oxide fine particles; and separating the transition metal oxide fine particles from the mixed aqueous solution, and washing, drying and heat-treating the separated transition metal oxide fine particles.
 15. The method of producing the transition metal oxide fine particles of claim 14, wherein the surfactant comprises at least one selected from a cationic surfactant, an amphoteric surfactant, an anionic surfactant and a nonionic surfactant, and the added surfactant has a mass ratio of 0.5 g or less per 1 L of water.
 16. The method of producing the transition metal oxide fine particles of claim 14, wherein the transition metal oxide comprises at least one selected from a group consisting of a molybdenum trioxide and a tungsten trioxide, and the dissolving of the transition metal oxide into the strong base aqueous solution includes sequentially dissolving the molybdenum trioxide and the tungsten trioxide into the strong base aqueous solution.
 17. The method of producing the transition metal oxide fine particles of claim 14, wherein the dissolving of the transition metal oxide into the strong base aqueous solution; includes agitating the transition metal oxide at a speed within a range of 100 rpm to 300 rpm and heating the transition metal oxide at a temperature within a range of 50° C. to 100° C., until the strong base aqueous solution become transparent.
 18. The method of producing the transition metal oxide fine particles of claim 14, wherein the forming of the mixed aqueous solution includes: adding the strong acid aqueous solution to the strong base aqueous solution; stopping the adding of the strong acid aqueous solution to the strong base aqueous solution based on the solids being generated at the interface; agitating the mixed aqueous solution until the solids are dissolved into the mixed aqueous solution; and again adding a strong acid aqueous solution to the strong base aqueous solution when the solids are dissolved into the mixed aqueous solution, and wherein the strong acid solution comprises 60% to 75% nitric acid.
 19. The method of producing the transition metal oxide fine particles of claim 14, wherein the drying of the transition metal oxide fine particles includes: primarily drying the transition metal oxide fine particles at a room temperature for at least 24 hours; and secondarily drying the transition metal oxide fine particles in a vacuum environment at a temperature higher than the room temperature, and wherein the heat-treating of the transition metal oxide fine particles; includes drying the transition metal oxide fine particles separated from the mixed aqueous solution, and then heat treating the transition metal oxide fine particles at a temperature within a range of 380° C. to 450° C. for at least 30 minutes.
 20. The method of producing the transition metal oxide fine particles of claim 14, wherein after the heat-treating of the transition metal oxide fine particles, the transition metal oxide fine particles are made of MoWO6 having a structure of Chemical Formula 1 below: 